The Caerphilly byelection only looks local if you ignore what it represents: the quiet unravelling of the democratic world’s most successful election-winning machine. For more than a century, Labour has been the dominant party in Wales. Its grip had survived deindustrialisation, Thatcherism and even the slow death of its working-class institutions. Now that seems to be ending – not through scandal or drama, but through a lack of moral imagination.

What happened in this small valleys seat, where Labour lost a heartland base to Plaid Cymru while Reform UK menaced from the right, is the result of a Downing Street mentality that mistakes competence for conviction and caution for strategy. Welsh Labour has not helped itself – vicious infighting had left it fractured and demoralised. As Labour controls both the Welsh and UK parliaments, there’s nowhere to hide. The trouble is that Sir Keir Starmer promised change and delivered continuity. Caerphilly looks like a reckoning.

The Senedd constituency – situated between Cardiff’s commuter south, the valleys’ post-industrial fringe and the Welsh-speaking west – has long been regarded as “safe Labour”. Its loss therefore carries an outsized symbolism. It suggests that the institutional glue of Labour Wales – geography, class and community – has dried out and cracked.

Plaid’s victory shows that Welsh politics can still produce moral energy. But clearly no longer through the Labour machine. Its brand of politics appears, for the first time, a vehicle for voters who want fairness without nostalgia – who feel Welsh rather than Westminster-facing, and progressive rather than populist. In that sense the voters of Caerphilly have ceased to speak Labour’s language because Labour has ceased to speak theirs.

The Guardian columnist Will Hayward observed that many people seemed “more afraid of the opponents of immigration than of the immigrants themselves”. His point was that the public’s “common sense” is realigning itself around ethics rather than pure economics. That ought to worry Sir Keir, who has neither mastered the politics of the heart nor of the wallet. Reform UK’s rise shows how easily moral frustration can tip into nihilism. But its limits in Caerphilly also suggest that the rise of rightwing populism in Wales has been contained by a sense of decency. When offered a politics that feels local and humane, Welsh voters still prefer it to a doctrine of spite. Yet the arithmetic of devolution could give Reform 30 of 96 Senedd seats – a Welsh base for wider antipathy.

Conditions in Wales seem a microcosm of the British crisis – just as at the turn of the last century, when the nation hosted a contest between industrial capitalism, nonconformist moralism and working-class dissent. More than a century ago, Welsh miners turned from chapel Liberalism to socialism, dramatising the shift from moral to materialist radicalism that would reshape Britain at large.

The upheavals of legitimacy, production and identity that define Britain today are all present in Wales. There’s a broken industrial economy replaced by low-wage, de-unionised service jobs; a party in power committed to managerialism, and a choice of either reactionary or progressive politics. The voters who turned from Labour to Plaid weren’t just switching parties; they were searching for a moral vocabulary that could also protect their material lives. That’s the terrain on which Plaid’s future – and perhaps the British left’s renewal – will be decided.